The use of laser scanning techniques for printing information on laser sensitive mediums have been disclosed in the prior art. For example, U.S. Pat. No. 3,922,485 disclosed a multifaceted polygon optical scanner which scans a modulated laser beam across a xerographic medium. In order to print on the laser sensitive medium (i.e. the xerographic drum shown in the aforementioned patent), a laser of a particular output power is required. For example, the photoreceptor which comprises the xerographic medium disclosed in the aforementioned patent requires a laser flux of one milliwatt incident thereon to discharge predetermined charged areas of the photoreceptor to accomplish printing. In order to reduce the power requirements on the input laser which in turn, would reduce its cost and size, the prior art has sought to optimize laser efficiency or, in other words, the efficiency of the optical system such that maximum laser beam power is provided on the photoreceptor for a given input laser rated at a certain output power. One approach has been the optimization of the key components which comprise the optical system such as the modulator, polygon scanner and other major optical elements. However, the optical system reaches a certain point where efficiency does not increase. It has been found that typically optical scanning system efficiencies are on the order of ten percent so that a ten milliwatt laser is required to apply one milliwatt of power on the photoreceptor. The impact of this performance is to require system designers to stress the laser power capability which in turn can effect the projected reliability, life, manufacturing cost, development cost, and field operational costs.
It should be noted that the inefficiency of some of the components in the system is due to the contamination of various optical surfaces as well as glass-air interface light power losses. The surface losses of each optical element in the system effects the transmission of each element and cumulatively effects the efficiency of the overall scanning system. Further, in scanning systems which require more than one facet to be illuminated in order to reduce retrace times, such as that disclosed in the aforementioned patent, reduced system efficiencies are the result since only one beam from one facet can be utilized at a time. Generally, in order to provide a relatively uniform amount of light across the scan line, the beam illuminating the scanner facets is expanded to fully illuminate the facets. The end result of the beam expansion is that the percentage of light which can get through the scanner, even if the surfaces thereof were perfect reflectors, is severely reduced. The problem inherent in illuminating two facets could be minimized by using a scanner facet dimension large compared to the optical beam at the polygon in the scan direction. Although this may be viable in a low resolution system or for a low speed scanner which can tolerate a large polygon dimension, this approach cannot be tolerated for high resolution systems or for high speed scanners.
The aforementioned disadvantages have been corrected by the system disclosed in copending application Ser. No. 785,258 filed Apr. 6, 1977, now U.S. Pat. No. 4,170,028, and assigned to the assignee of this application. As disclosed therein, an active optical element is utilized to deflect the incident laser beam so as to follow one facet during a complete scan and shift to the next facet for the following scan. The active optical element in low and high bandwidth systems preferably is an acousto-optic Bragg cell used to both modulate and deflect an incident laser beam.
Unless compensated for, motion blur problems can arise in those forms of optical data records, such as the laser scanning systems described hereinabove, in which there is significant relative movement between the recording medium and the focused laser writing beam incident thereon. Reduction of motion blur by the use of very fast electro-optic modulators is possible, but that technique tends to be rather costly. State-of-the-art acousto-optic modulators are not effective in many potential applications because of the practical limitations in the rise time of the modulator which is imposed by the transit time of the acoustic wavefront across the laser beam, thereby reducing or severly limiting the response of the modulator to high speed input video information. A technique for reducing the bandwidth and rise time limitations associated with the use of state-of-the-art acousto-optic modulators in an optical data recording system by reimaging the motion of the acousto-optic pulse onto a recording medium thereby greatly increasing the effective bandwidth of the acousto-optic modulator and reducing any blurring of the image formed on the surface of the recording medium is disclosed in copending application Ser. No. 920,314, filed June 28, 1978. In the embodiment in which a rotating scanner device and a xerographic recording medium are utilized, selection of the system magnification between the modulator and the recording medium to be subtantially equal to the ratio of the velocity of the scanning laser writing beam, to the velocity of the acoustic wave front in the acousto-optic modulator causes the acoustic pulse (which essentially contains the video information) to be reimaged onto the surface of the recording medium in a manner whereby the acoustic pulse follows the recording surface and permits an isomorphic mapping of the video signal to the recording medium without blurring.
The first concept described hereinabove enables high optical throughput efficiency to minimize the required laser power. However, system resolution performance, although satisfactory for most purposes, is less than desired in applications which require high resolution in the output produced by the system. It should be noted that the resolution in prior art scanning systems which do not utilize the facet tracking concept are minimally affected by the inherent pulse imaging presence occuring in acousto-optic modulators, the effects being capable of being compensated for by techniques known to the prior art. However, when facet tracking is utilized, the effects become important and should be utilized to improve system resolution. Additionally, existing acousto-optic modulators are effectively non-responsive to very high video data rates.
The second concept (without facet tracking) described hereinabove improves the system resolution by minimizing image blur and reduces sensitivity of the system to laser beam wander. However, the translational motion of the polygon facet introduces a distortion of the laser beam exposure profile which varies along the length of the scan line being recorded, the output copy being produced thereby being less than ideal.
It has been determined that although both concepts can be utilized separately in laser scanning applications and provide satisfactory results, the combination of both concepts in the same laser scanning system enables system performance to be substantially upgraded to a degree which cannot be realized by implementing each concept separately.